DPAC venture

[Rusted]

3 hours ago, jetstream said:

So then the zones would all be in the same place relative to the optic making them identifiable ?

Exactly. Like moving to a clearer spot on a distorted window.
One could test a larger optic with a planned series of movements of the optical flat. Or of the test objective around the smaller flat.
Just as one can stitch together a larger picture from smaller areas of the whole image. [Like an imaging mosaic.]

[davidc135]

13 hours ago, Rusted said:

One can eliminate some zoning on the optical flat by setting the flat off centre to the test objective.
It does not make the zones go away, of course. It just shows them for what they are.
So the objective, under test, is not blamed for the errors on the flat itself.

Yes, it's true that zones would be concentric with the faulty optic. More tricky would be where there is a smooth variation in the 'flat's' curvature giving an over or under-correction of 1/5th wave to the reflected wavefront.

How a possible combination of zones and over/under-correction due to the flat would interact with the same due to the optic, even offset, I'm not sure. 

It's well worth using an erf if you already have one and seeing what happens. Otherwise I'd sooner buy a well speced ex Soviet flat from Ukraine for a modest amount. Not being in a cell means that it can be checked with a water test. See the video by Ed Jones on Youtube.

If the filter is removable (?) it too could be checked

David

Edited March 20, 2023 by davidc135

[davidc135]

What complicates buying flats is knowing whether they are being rated by their surface accuracy or that of the reflected wave front. It should be their surface but it's often not clear. To be safe it should be explicit.

David

Edited March 20, 2023 by davidc135

[Highburymark]

The Baader ERFs are highly rated. Speaking of enthusiastic SV marketing, Baader’s blurb for its ERFs describes them as “hellishly diffficult” to make, with “the world’s most advanced coating machines, inspected on a Carl Zeiss optical bench. 1/10 wave”……. so should be as good as any ERFs.  
 

[davidc135]

In favour of any 1/10 wave flat would be:

If the tested optic seems perfect then it should be correcting an at worst 1/5 wave error due to the flat. But the tested optic's errors are doubled due to double pass and so, in theory, it should only have a max of 1/10 wave error.

David

[davidc135]

And possibly against the erf is:

If the erf's accuracy is rated at 1/10 wave is that surface or transmitted wavefront? If the latter, the surface error is now doubled due to refraction. Very likely they are fine but it would be good to see how they tested against a known, quality objective.

David

[PeterW]

Anyone making large aperture etalons already has the kit and skills for getting the flatness sorted…..

 

Peter

[jetstream]

2 hours ago, Highburymark said:

The Baader ERFs are highly rated. Speaking of enthusiastic SV marketing, Baader’s blurb for its ERFs describes them as “hellishly diffficult” to make, with “the world’s most advanced coating machines, inspected on a Carl Zeiss optical bench. 1/10 wave”……. so should be as good as any ERFs.  
 

Does Baader make the Solar Spectrum etalons? I might be copying your set up with the TSA120

Maybe an email to Baader might reveal the addition specs, Ive found them great in the past,

[Highburymark]

24 minutes ago, jetstream said:

Does Baader make the Solar Spectrum etalons? I might be copying your set up with the TSA120

Maybe an email to Baader might reveal the addition specs, Ive found them great in the past,

No it doesn’t Gerry - Solar Spectrum is a US company - they are just marketed by Baader. Also the Baader Sundancer (Quark type filter) incorporates a Solar Spectrum etalon, but the etalons are all made in the States. Very nice quality on the whole - but PM me before you press any buy buttons. 

[jetstream]

6 minutes ago, Highburymark said:

No it doesn’t Gerry - Solar Spectrum is a US company - they are just marketed by Baader. Also the Baader Sundancer (Quark type filter) incorporates a Solar Spectrum etalon, but the etalons are all made in the States. Very nice quality on the whole - but PM me before you press any buy buttons. 

Will do- I'm hoping to re do the Ha situation here- I have a Quirk atm, works ok in a small band but far far from optimum. Thanks Mark 👍

[jetstream]

3 hours ago, davidc135 said:

And possibly against the erf is:

It would sure be nice to incorporate the D ERF into both DPAC and also into the platform for Ha with the TSA120, then the cost is justifiable. Just looking at Ceravalos paper on intereferometry and the construction of one... maybe thats next lol

Do you have an interferometer?

[davidc135]

No interferometer as yet. The 6'' and 12'' flats are nice but other than those, my kit is very basic.  David

[jetstream]

9 hours ago, davidc135 said:

No interferometer as yet. The 6'' and 12'' flats are nice but other than those, my kit is very basic.  David

Baby steps for me first but I might build one to see if I could do it and also to see if it would be accurate. Real, high spec flats are $12000 USD though 😔

[skybadger]

Not using an oil bath then, for 2c ?

[jetstream]

4 hours ago, skybadger said:

Not using an oil bath then, for 2c ?

At this time the oil bath is looking like a great method!